Who really invented the sextant?

In the archives of the Royal Society in London there are two affidavits, both sworn on the 27th of March, 1733, before Samuel Hasell, a justice of the peace for the city of Philadelphia.

The first affidavit, sworn by Edmund Wooley, carpenter, states that in or about November of 1730 he was employed by Thomas Godfrey to construct a special "sea-quadrant" to Godfrey's specifications.

The second affidavit, sworn by George Stewart, a mariner, states that about the end of October 1730, Godfrey described to him an instrument he had developed that allowed the operator, while at sea, to measure accurate altitudes of the sun or distances between celestial objects with the help of double reflections. Stewart further describes how Godfrey modified one of his standard Davis quadrants by the addition of two pieces of "looking glass."

Thomas Godfrey, born at Philadelphia in 1704, was gifted in intellect but poor in resources. He had a humble education. He was able to read and write and perform simple arithmetic, but, lacking money to finance a higher education, he became a glazier to support his family. Godfrey was employed for a time installing window glass at the Philadelphia State House (now Independence Hall). It is said that the idea for his double-reflecting instrument occurred to him early in his apprenticeship while handling two pieces of glass and noticing the effects of their combined reflections.

As years passed, Godfrey apparently neglected his trade and family to pursue a self-education in mathematics and astronomy, eventually exhausting his supply of books in English, whereupon he taught himself to read Latin.

It was Godfrey's pursuit of Newton's book, Principia Mathe-matica (written in Latin), that brought him into contact with James Logan, friend and confidant of Benjamin Franklin. Logan befriended the bright young man by allowing Godfrey the use of his well-stocked library. Godfrey, Franklin, and Logan were three of the original founding members of the Junto Club, a philosophical and scientific coffee klatch peopled by the best minds in Philadelphia. Franklin had rented rooms to Godfrey in his own home in 1727, but they appear to have had a falling out. Franklin may have harbored some jealousy of Godfrey's growing intellect as evidenced by the following description of Godfrey in his memoirs: Godfrey was "a self-taught mathematician, great in his way" but who "knew little out of his way, and was not a pleasing companion, as like most great mathematicians I have met with, he expected precision in every thing, and was ever denying and distinguishing upon trifles, to the disturbance of all conversation."But the failure to establish a long-term friendship with Franklin did not divert Godfrey's inquiring mind from pursuing his invention. In October of 1730, the mariner, Stewart, brought Godfrey one of his quadrants to have it modified and reflecting glasses installed. The great advantage of using the double reflecting system allowed the observer to carefully determine the altitude of the sun or other celestial body regardless of a ship's motion. On November 28, Stewart and his modified quadrant set out on a voyage to Jamaica aboard the sloop Truman, where he was mate. During the voyage he made numerous sights with his instrument, finding it very accurate in altitude measurements (each marked degree being read at double the reading because of the installation of the double mirrors). In February of 1731 Stewart again set sail, this time for Saint John's, Newfoundland. During this voyage both Stewart and his captain, John Cox, found measurements made with the modified quadrant "very correct."Godfrey also gave John Logan the device that Edmund Wooley had constructed from Godfrey's plans in 1730, and Logan later recalled that "he told me he had for some time been thinking of an instrument for taking the distance of stars by reflecting speculums [polished surfaces], which he believed might be of service at sea."

Godfrey left the idea and the special quadrant with Logan at the end of 1730 so that his friend might contact the Royal Society at London to advise that august group of his invention. Unfortunately for Godfrey, Logan was soon after appointed chief justice. His new responsibilities took most of his time, and so it was not until 18 months later (May 1732) that Logan finally wrote to Dr. Edmund Halley (of Halley's Comet fame) at the Royal Society, describing the essential parts of Godfrey's device (see upper diagram at left).

Logan's letter was not read before the Society for almost two years, being "lost or misplaced" for that considerable length of time. Finally on January 31, 1734, Logan's letter and an additional one from Godfrey were read before the members of the Royal Society advising of the colonial discovery. However, Thomas Hadley (a member of the Royal Society) had already been given credit for the invention of the double reflecting sextant in 1731. All that the Society would admit was that both inventions had occurred independently on both sides of the Atlantic.

The device that Thomas Hadley displayed to the Royal Society, and for which he received praise as inventor, bore a great deal of similarity to Godfrey's design (see lower diagram on page 30).

My suspicion is that Godfrey's design may have been seen in Jamaica during Stewart's November 1730 visit by a navigator who advised the Royal Society and possibly Hadley of its superior performance. It would not be difficult to imagine a Royal Society that could not accept the idea that a colonial glass merchant might be given credit for the production of such an innovative and sophisticated device.

Thomas Godfrey, suffering from the effects of alcoholism, died in Philadelphia in poverty in 1749.

J. Gregory Dill is a freelance writer, an avid celestial navigator, and a marine antiquarian who lives in Dartmouth, Nova Scotia.